1. Field of the Invention
The present invention relates to a structure of protection of an integrated circuit against overvoltages due, for example, to electrostatic discharges. It more specifically aims at a structure for protecting a circuit connected to a differential data transmission line.
2. Discussion of the Related Art
FIG. 1 is a very simplified electric diagram of a device comprising an integrated circuit IC1 capable of communicating, via a differential transmission line or connection 10, with a device, not shown. Link 10 comprises two rails 11 and 12, respectively connected to terminals 13 and 14 of circuit IC1. In operation, data signals of the same amplitude but in phase opposition flow through rails 11 and 12. This link type is generally used for broadband data transmission.
FIG. 2 is a timing diagram schematically illustrating the variation, during the device operation, of signals 21 and 22 on the respective rails 11 and 12 of link 10 of FIG. 1. In this example, signals 21 and 22 are digital signals capable of taking one or the other of two values VMIN and VMAX, respectively high and low, centered on an average positive voltage Vm·ΔV will be used to designate the excursion of signals 21 and 22, that is, ΔV=VMAX−Vm=Vm−VMIN. As illustrated in the drawing, signals 21 and 22 are of same amplitude but in phase opposition.
Signals 21 and 22 generally are signals with a low voltage level. As an example, average voltage Vm may range between 0.1 and 5 V and, in most current differential transmission standards, excursion ΔV ranges between 0.1 and 0.6 V. Thus, value VMAX is generally smaller than 6 V. However, abrupt temporary overvoltages or over currents, of high amplitude, capable of damaging components of integrated circuit IC1 (FIG. 1), may occur on rails 11 and/or 12 of the differential link. Such overvoltages may occur while circuit IC1 is powered, during the normal operation of the device, or when circuit IC1 is not powered, for example, during assembly phases of the device.
It is generally provided to connect to differential link 10, between rails 11 and 12, a protection structure 15 (FIG. 1), capable of rapidly draining off significant currents that may appear when an overvoltage or electrostatic discharge occurs on rail 11 and/or on rail 12.
FIG. 3 is an electric diagram of a conventional example of protection structure 15 of FIG. 1. Structure 15 comprises a diode bridge comprising a diode 31 in series with a diode 33 and, in parallel, a diode 35 in series with a diode 37. A diagonal of the bridge is connected between rails 11 and 12. The other diagonal of the bridge is connected to a zener diode 39 having a grounded terminal.
It should be noted that term “ground” here designates a reference potential common to all the device elements, for example, a potential close to 0 V. In practice, structure 15 may be grounded via a ground terminal of integrated circuit IC1, or via a ground rail (not shown) comprised in link 10. In the following description, “positive potential” and “negative potential” will be used to designate, respectively, potentials greater than the ground potential and smaller than the ground potential, and each time digital potential values will be given as an example, it will be considered that these values refer to a ground potential equal to 0 V.
In case of a positive overvoltage on rail 11, if the overvoltage exceeds a given threshold, zener diode 39 becomes conductive by avalanche effect, and the overvoltage is removed towards the ground, via diode 33 and zener diode 39.
In case of a negative overvoltage on rail 11, diode 31 becomes conductive and the overvoltage is removed by this diode.
Similarly, in case of a positive overvoltage on rail 12, zener diode 39 becomes conductive by avalanche effect and the overvoltage is removed via diode 37 and zener diode 39. In case of a negative overvoltage on rail 12, the overvoltage is removed via diode 35.
Thus, structure 15 enables removing any overvoltage likely to occur on rails 11 and/or 12. The turn-on threshold for a positive overvoltage is equal to the forward voltage drop VF of a diode (on the order of 0.6 V) plus the avalanche voltage of zener diode 39. The turn-on threshold for a negative overvoltage is equal to the opposite of the forward voltage drop of a diode (on the order of −0.6 V).
Avalanche diode VBR of zener diode 39 must be greater than voltage VMAX−VF. This actually results, to take into account the component dispersion, in selecting an avalanche voltage much greater than VMAX. This is a first disadvantage of such a protection structure.
Another disadvantage of this type of structure is that if circuits adapted to different levels VMAX are desired to be protected, zener diodes of low avalanche voltage should be provided to properly protect circuits adapted to signals of low level VMAX. Thus, it should be provided to manufacture multiple specific protection structures adapted to the different circuits to be protected.